1. Field of the Invention
The present invention relates to monochromators and, more particularly, to a high-accuracy scanning monochromator with improved optical path and direct drive diffraction grating for constituent or characteristics analysis of a sample.
2. Description of the Background
Scanning monochromators are well-known and generally include an oscillating diffraction grating, a drive mechanism for rotating the diffraction grating, a light source, and light sensor. Light from the light source is dispersed off the diffraction grating and the narrow bands of diffracted light are scanned across a sample. An encoder tracks the angular position of the grating, and this coupled with a measurement of the narrow bands of light reflected off the sample gives an absorption spectrum by which the sample can be analyzed.
In oscillating the diffraction grating, the drive assembly must be capable of producing a linear change in wavelength at the exit aperture of the monochromator. Unfortunately, the efficiency of diffraction gratings are not constant as they oscillate (efficiency being defined as the power of monochromatic light diffracted into the order being measured, relative to the energy flow of the incident light). Thus, it becomes necessary to control the magnitude and variation of diffracted energy with wavelength. This control process is known as “blazing”, and it is traditionally accomplished by manipulating the geometry of the grating grooves across the face of the grating. R. W. Wood pioneered blazing by producing the first grating having a controlled groove shape. This facilitated modern diffraction gratings which have specific efficiency curves. The choice of an optimal efficiency curve for a grating depends on the specific application. In the context of a monochromator, the desired instrumental efficiency is linear. In other words, the intensity of light transformed into signals at the image plane must be constant across the spectrum. This requires peak grating efficiency in the region of the spectrum where the detectors are least sensitive. For example, a visible-light spectrometer using a silicon detector would be much less sensitive in the blue than in the red, and thus the grating must be blazed to yield a peak efficiency in the blue. A typical efficiency curve shows a single maximum, at a peak wavelength λB. The efficiency curve also depends on the angles of use (i.e., the angles of incidence and diffraction). Moreover, the curve depends on the groove spacing and the material with which the grating is coated.
Traditionally, diffraction gratings have been rotated or oscillated by a drive motor through a mechanical reduction drive assembly. However, the diffraction grating must be rotated sinusoidally with extreme accuracy, and it is very difficult to implement a drive that can achieve this with the requisite resolution. Moreover, these drive assemblies leave no convenient means of calibration.
For instance, U.S. Pat. No. 5,096,295 to Krupa et al. illustrates a reduction gear arrangement wherein the diffraction grating is mounted to a rotatable platform and is motor-driven via a reduction gear transmission. A Hall-effect sensor is used to track the angular position of the diffraction grating. Unfortunately, neither the Hall-effect sensor or any other known encoders are capable of high-resolution tracking as necessary to compensate for mechanical tolerances. Even the slightest tolerances in the mechanical parts can dramatically affect accuracy and performance of existing drive systems. Moreover, mechanical configurations lead to mechanical wear, and this too affects accuracy and performance over time. A direct-drive diffraction grating and control system would provide much more flexibility and programmable control, as well as providing the ability to compensate for mechanical tolerances. However, it has not previously been possible to attain the degree of resolution required in a direct-drive diffraction grating system using conventional servo or stepper motors.
U.S. Pat. No. 5,625,270 to Barker et al. shows a scanning monochromator with a directly driven spectral-dispersion element. The spectral-dispersion element is directly coupled to the output shaft of a stepper motor. The movement of the motor is controlled with an electronic circuit that switches electrical current through the motor windings and produces a variable pulse-width ranging from 0 to 100 percent modulation. The electrical circuit includes pulse-width modulator (PWM) and computer sections. The PWM section accepts values from the computer section and converts these values into variable pulse-width signals for use by the power driver section. The computer section supplies values to the PWM section based upon requirements for the motor movement and position. The required motor position is determined by the type of spectral-dispersion element and the desired wavelength selection of the monochromator. However, the stepper motor relies on two-phase windings, and the pulse-width modulation is accomplished by controlling the ratio of the electrical currents applied to the two windings. This method of pulse-width modulation entails complex control calculations and commensurate hardware, again too slow and costly for a production environment.
It would be greatly advantageous to provide an economical scanning monochromator with direct drive diffraction grating and encoder feedback to a stepped control system, the control system being capable of pre-programmed introduction of local phase delays to adjust for aberrations in the optical system, thereby providing a linear composite wavefront and more accurate results.
In addition, it would be greatly advantageous to provide a more convenient and efficient method of calibrating the monochromator without sacrificing accuracy. The multiplicity of variables and the extreme resolution required in a scanning monochromator typically compels tight calibration standards. Traditionally, monochromators which calibrate based on reference scans using a standard sample (often polystyrene or rare earth powders) from the National Institute of Standards and Technology (NIST). This baseline scan is used as a reference to compensate subsequent live scans for component tolerances and slippage. However, it is very difficult to calibrate a monochromator this way in the field. As a result, prior art monochromators are factory-calibrated and then slowly lose their accuracy over time.
It is well recognized that anomalies occur along the efficiency curve at which the efficiency changes abruptly. First observed by R. W. Wood, these sharp peaks and troughs in an efficiency curve are referred to as Wood's anomalies. Lord Rayleigh suggested that anomalies occur when light of a given wavelength and spectral order is diffracted at 90° from the grating normal (i.e., it passes over the grating horizon). This results in a discontinuity in the diffracted power for a given wavelength and order because the power that would diffract into the given order is instead redistributed among other spectral orders. This causes abrupt changes in the power diffracted into the other spectral orders.
U.S. Pat. No. 4,330,211 to Peterson suggests a method and apparatus for compensating for Wood's anomalies using a second diffraction grating. Any deviation from the Wood's anomaly region by the input beam will substantially reduce the intensity of the output. This intensity variance is detected and utilized as an indication of a small angular deviation of the input beam. Peterson suggests the utilization of a control system to continually maintain the doubly diffracted beam intensity or efficiency at a maximum, thus allowing the direction of an incoming beam to be maintained to a high degree of precision. However, this approach leads to a complex optical system and a commensurate control system, too slow and costly for a production environment.
Despite the foregoing, Wood's anomalies provide just as much opportunity as obstacle. If a processor is programmed to resolve the position of the Wood's regions, the data can then be compared against baseline stored values established at factory calibration. In other words, calibration can be achieved automatically using the Wood's regions as opposed to prior art monochromators which are calibrated based on reference scans using a standard sample from the NIST. This would be greatly advantageous because the monochromator could be calibrated frequently (i.e., at time intervals ranging from just prior to every live scan to every ten or twenty minutes, or longer). Moreover, calibration can take place in the field without the need for expensive sampling standards.